You'll need the following dependencies:

• libchamplain-0.12-dev
• libchamplain-gtk-0.12-dev
• libclutter-1.0-dev
• libecal1.2-dev
• libedataserverui1.2-dev
• libfolks-dev
• libgee-0.8-dev
• libgeocode-glib-dev
• libgeoclue-2-dev
• libglib2.0-dev
• libgranite-dev >= 6.2.0
• libgtk-3-dev
• libical-dev
• libhandy-1-dev >= 0.90.0
• meson
• valac

Run meson build to configure the build environment. Change to the build directory and run ninja to build

meson build --prefix=/usr
cd build
ninja

To install, use ninja install, then execute with io.elementary.calendar

sudo ninja install
io.elementary.calendar